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Chapter 9-10
The Reproduction of Cells
Mitosis & Meiosis
Cell Division

“Omnis cellula e
cellula” – Virchow
– “all cells from cells”

Cells reproduce to
form genetically
identical daughter
cells
Functions of Cell Division

Reproduction:
– Making more cells

Growth:
– Enables multicellular organisms to grow and
develop from a single cell

Repair:
– Replacing cells that die from normal wear and
tear and accidents
Prokaryotic Cell Division

Binary Fission:
– “division in half”

Steps in Binary
Fission
– Bacterial DNA is
duplicated
– Cell grows
– Cell splits in two
Eukaryotic Cell Division

The Cell Cycle
– Interphase
 G1, S, and G2 phases
– Mitotic Phase
 Mitosis
 Cytokinesis
Interphase


The cell spends 90% of
its time in interphase
G1:
– “Gap 1”
– Cell growth

S:
– “Synthesis”
– DNA is replicated

G2:
– “Gap 2”
– Cell growth
– Preparation for division
Mitotic Phase

5 subphases:
– Prophase
– Prometaphase
– Metaphase
– Anaphase
– Telophase
Prophase
Nucleoli disappear
 Chromatin condense
into chromosomes
 Mitotic spindle begins
to form in the
cytoplasm
 Centrosomes move
away from each other

Prometaphase
Nuclear envelope
disappears
 Bundles of
microtubules extend
from each pole
towards the equator
of the cell
 Some microtubules
attach to kinetochores

Metaphase
Centrosomes are at
opposite poles
 Chromosomes line up
on metaphase plate
 Mitotic spindle is fully
formed at this point

Anaphase
Sister chromatids
separate at the
centromere
 Chromatids move
towards the poles
 Cell becomes more
ovoid in shape

Telophase
Daughter nuclei begin
to form at the poles
of the cell
 Nuclear envelopes
reform
 Nucleoli reappear
 Chromotin becomes
less dense 
chromosomes
disappear

Cytokinesis
Division of the
cytoplasm
 Animal Cells:

– Cleavage furrow forms
– Pinches the cell into
two

Plant Cells:
– Cell plate forms
– Divides the cell in 2
Animal Cells
Plant Cells
Controlling Cell Division
Cells rely on internal and external cues to
control cell division
 Growth factor

– Substance that is required for certain cells to
divide

Density of cells
– If cells become too dense in their container,
they will stop dividing  they’re competing
for resources!
Restriction Point

Late in G1, before S phase, the cell passes
the “point of no return”
– Can we go ahead and divide?
– If yes… cell division occurs!
– If no, the cell enters G0 phase
 Essentially exits the cell cycle and just stops
 Most grown human cells are in this phase
MPF

“Maturation Promoting Factor”
– A specific type of cyclin-dependent kinase
(cdk)
– Cdks are regulatory proteins
 Helps control the cell cycle
– The “master switch” for a cell’s passage from
interphase to mitosis
Control of the Cycle



The phases are
triggered by the
accumulation of
control substances
called cyclins.
The cell division
cyclins interact with
molecules called
kinases.
Kinases are proteins
that phosphorylate
other chemical
messengers or
enzymes that trigger
the cell cycle phases.
Control of the Cycle
The cyclin-dependent kinase, cdk,
has
the ability to activate either a cellular
messenger for either DNA replication or
mitosis. The messengers are called
cyclins. M-cyclin for mitosis
and the Scyclin for DNA synthesis
Control of the Cycle

In G1 the cdk is
inactive.
Control of the Cycle

The S cyclin protein
forms.

Levels of cyclins can
change, but kinases
cannot.
Control of the Cycle

S cyclin combines
with the cyclindependent kinase
which is in it DNA
promoting (S) form.
Control of the Cycle
The messenger
molecule attaches to
the activated cdk.
 The messenger is
phosphorylated and
then breaks away
from the cdk.
 S begins.

Control of the Cycle
The messenger
leaves.
 The S-cyclin is
destroyed.
 The cdk (cyclindependent kinase) is
again inactive.

Control of the Cycle
At end of S, the cdk
will change to the M
form (mitosis
promoting form).
 M- cyclin will be
produced.
 M-cyclin will attach
to the M form of cdk
to form a MPF
(maturation
promoting factor).

Control of the Cycle




The signal molecule can
attach and then
becomes
phosphorylated.
The signal molecule will
detach, signaling
mitosis.
M-cyclin is destroyed.
Cyclin-dependent kinase
(cdk) returns to inactive
form.
Control of the Cycle





Three checkpoints exist (G1, G2, and during
metaphase of M).
Checkpoints are regulated by amount of cyclins (
in different forms).
The most important checkpoint is in G1
(RESTRICTION POINT).
If enough cyclins are present the cell goes
through the rest of the cycle.
If not, the cell stops dividing and enters G0.
Normal v. Cancer Cells
Have densitydependent inhibition
 Will only divide when
attached to a
substrate (anchored)
 Must have sufficient
growth factors or
nutrients for division

Lack densitydependent inhibition
 Will divide in solution
or when not anchored
 Unaffected by
presence of external
growth factors or
nutrients

Cancer

Tumor:
– When cancerous cells divide, they form a mass of
cancer cells within an otherwise normal tissue

Benign tumor:
– Cancerous cells remain in one spot

Malignant tumor:
– Becomes invasive enough to impair the functions of
one or more organs

Metastasis:
– The spread of cancer cells beyond their original site